In recent years, advances in technology, as well as ever evolving tastes in style, have led to substantial changes in the design of automobiles. One of the changes involves the complexity of the electrical and drive systems within automobiles, particularly alternative fuel vehicles, such as hybrid, electric, and fuel cell vehicles. Such alternative fuel vehicles typically use an electric motor, perhaps in combination with another actuator, to drive the wheels.
Hybrid and electric vehicles (HEVs) typically include an electric traction drive system that includes at least one alternating current (AC) electric motor which is driven by a power inverter module (PIM) with power from a direct current (DC) power source, such as a storage battery. Motor windings of the AC electric motor can be coupled to inverter sub-modules of the PIM. Each inverter sub-module includes a pair of switches that switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. A pulse width modulation (PWM) module receives voltage command signals and applies PWM waveforms to the voltage command signals to control pulse width modulation of the voltage command signals and generate switching vector signals that are provided to the inverter sub-modules of the inverter module. When the switching vector signals are applied, each pair of switches in each of the inverter sub-module switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. This AC power drives the AC electric motor, which in turn drives a shaft of HEV's drivetrain.
Vector Control
Many modern high performance AC motor drives use the principle of field oriented control (FOC) or “vector” control to control operation of the AC electric motor. In particular, vector control is often used in variable speed drives to control currents fed to an AC electric motor so that mechanical angular velocity of motor's rotor can be controlled and hence the torque applied to a shaft by the AC electric motor can be controlled. In short, stator phase currents are measured and converted into a corresponding complex space vector. This current vector is then transformed to a coordinate system rotating with the rotor of the AC electric motor.
Vector control requires angular position information for the rotor (i.e., the mechanical rotational angular position of rotor relative to the “stator” or motor windings). Angular position information is normally obtained via a feedback device (e.g., angular position or speed sensor). However, in some systems, sensorless control techniques can be used to provide angular position and/or angular frequency/speed information.
Sensor-Based Control
Traditional motor control systems normally include a feedback device to provide angular frequency (or “speed”) and angular position information about the motor. Many vector controlled motor drive systems employ a rotor speed or position sensor to provide information regarding the rotor's angular position that is needed to control the motor. For example, the rotor's angular position can be computed based on actual measured quantities using some type of speed or position sensor for control feedback measurement. For instance, to determine the angular position of the rotor, its angular speed can be measured with a speed sensor, and the angular position can then be obtained by integrating the speed measurements. Other field-oriented or vector controlled systems may use a rotor angular position sensor or rotational transducer that provides absolute position information directly to implement motor control techniques. One such example would be a resolver and resolver-to-digital converter circuit, which directly provides position information that corresponds to the rotor's angular position.
The cost of feedback devices and associated interface circuits is significant. Removal of a feedback device for an electric motor control system (and its associated wiring and circuitry) can reduce the cost of an HEV. As such, it is desirable to eliminate this speed and/or position sensors and replace the measured quantities by computed estimates in some HEVs. In some systems, a speed or position sensor may not be implemented, and sensorless control techniques, which are described below, can be used to estimate angular position or frequency/speed.
Position/Speed Sensor Fault
In some operating scenarios, the speed or position sensor may not operate as intended (e.g., during a fault). For example, in some cases a sensor can experience a fault or fail in which case measurements provided by the sensor will be incorrect or missing completely. For instance, a loss-of-tracking (LOT) failure can result, for example, when the motor is operating in its overspeed region and the rotor's angular velocity (or “motor speed”) exceeds a tracking threshold limit of the position sensor. Alternatively, LOT failure can also result, for example, when an internal position error of the position sensor exceeds a certain preset threshold. When a position sensor experiences a LOT failure, the rotor angular position measurements that are normally provided by the position sensor will usually be incorrect or missing completely. As such, unless there is a way to estimate angular position or speed, it is likely that field-oriented vector control techniques will not work as intended and it becomes necessary to shutdown the electric motor drive since it relies on this information to ensure correct operation.
Sensorless Control
As alluded to above, the objective of the sensorless control is to obtain the rotor angular position information without using speed or position sensors to measure actual speed/position quantities. Instead, electromagnetic characteristics of an AC machine can be used to eliminate the need for such sensors and their associated wiring and interface circuits. Numerous methods for estimating the rotor's angular position and speed have been developed. The methods can generally be classified as those that work better at low motor operating speeds and those that work better at high motor operating speeds. One approach that can be used to cover the entire range of motor operating speeds (i.e., rotor angular velocities) is to selectively enable/disable two different sensorless control techniques depending on the motor speed. For example, at high motor operating speeds, one sensorless control technique can be enabled, and at low motor operating speeds (or zero speed), a different sensorless control technique can be enabled.
Responses During a Fault Condition: Three-Phase Short Versus Open Response
As noted above, during normal operation the switches in each inverter module are operated in a complementary manner such that when one is switched on, the other is switched off, and vice-versa. However, during many different types of fault conditions, it is desirable to deviate from normal complementary operation and to apply either an open-circuit fault response or a short-circuit fault response at the inverter module to minimize the electric machine's torque response.
During an open-circuit response, all switches in the multi-phase inverter are controlled to be open. For example, an open-circuit fault response can be applied at the inverter module by applying open-circuit fault response control signals to the PWM module that will cause an open-circuit fault response at the inverter module (i.e., cause all switches within the inverter module to be in an open state).
By contrast, during a short-circuit response, selected switches in the multi-phase inverter are controlled to connect all phases of the multi-phase inverter to a single bus (e.g., either the plus bus or the minus bus), and all other non-selected switches in the multi-phase inverter are controlled to be open such that the non-selected switches are not connected to the single bus (e.g., either the plus bus or the minus bus). For example, a short-circuit fault response can be applied at the inverter module by applying short-circuit fault response control signals to the PWM module that will cause a short-circuit fault response at the inverter module.
Whether an open or short-circuit fault response is applied at the inverter module can depend, for example, upon the machine's angular velocity (or “speed”). One approach for determining whether an open or short-circuit fault response is to be applied is disclosed in U.S. Pat. No. 7,279,862 B1 and Reissue Pat. RE 42,200, entitled “Fault Handling of Inverter Driven PM Motor Drives” assigned to the assignee of the present invention, their contents being incorporated by reference in their entirety herein.
Although the instantaneous angular velocity of the machine's rotor can often be determined based on the output of a position sensor (or read directly from a speed sensor), as described above, in some operational scenarios, these sensors may themselves experience a fault (e.g., when the speed/position sensor fails), and therefore, the particular instantaneous angular velocity can not be easily determined (e.g., read from a angular velocity sensor or determined from the position sensor). In other sensorless systems, such sensors are not implemented at all.
Existing techniques for estimating speed of an AC electric machine (e.g., a PMM) can be inadequate since it may be necessary to wait before an estimate can be computed.
It would be desirable to provide improved methods, systems and apparatus for estimating angular velocity (and/or angular position) of a rotor of a permanent magnet machine (PMM). It would also be desirable if such improved methods, systems and apparatus allow for angular velocity (and/or position) of the rotor to be estimated quickly during certain operating scenarios. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.